I'm going to echo what everyone else is basically saying, level up. It's a common theme in the Ys games, if a boss is kicking your ass, you need to go up a level or two. Sometimes going up two levels will take you from getting your ass kicked to holy crap that boss was a joke. In the entire franchise I can only think of two really frustrating as hell bosses that leveling doesn't help, and that's the last two bosses of Ys IV Mask of the Sun. Seriously, what the f*ck at them.